This invention relates to plasma CVD apparatus for forming thin films, including noncrystalline semiconductor films such as noncrystalline or amorphous silicon (hereinafter called a-Si for short), nitrate films such as SiN.sub.x (x=0-4/3) and oxide films SiO.sub.x (x=0-2), that can be obtained by glow discharge composition.
Heretofore, plasma CVD apparatus for manufacturing plates with thin films have included a high-frequency electrode and a substrate electrode for holding a substrate, mounted in face-to-face relation in a vacuum reaction chamber. To increase the number of substrates to be processed, a plurality of electrode pairs are alternately connected in parallel, substrates used to form the thin films are arranged on both sides of each of the substrate electrodes, and power is supplied to all the electrodes from a single power supply to provide a mass production type apparatus, as disclosed in Japanese laid-open application No. 48416/1983.
A thin film is normally formed by applying an AC electric field across the electrodes at a gas pressure of about 0.1 to 10 torr and decomposing a gas in the chamber by means of a glow discharge. Since a single power supply is used to apply the electric field to all electrodes, all of the electrode pairs are connected in parallel. If the size of the substrate is increased when this method is used, uniformity of the thin film formed on all of the substrates in an array cannot be attained at a frequency of 13.56 Mhz even though a film of good quality is normally obtainable at that frequency. On the other hand, if a frequency of approximately 100 KHz is used, the electrical stress applied to the film is very large and satisfactory film properties are not uniformly obtained when as a-Si film is formed. The reasons for the inability to obtain film uniformity, particularly at high frequency, are that isometric discharging is impossible because the load impedances of the electrodes are different from one another and, in addition, the discharge condition changes with time. When the substrates are arranged in an array another disadvantage is that discharging is unstable because of electromagnetic interference between the electrodes.
Accordingly, an object of the present invention is to provide an apparatus for manufacturing plates having thin films which overcomes the above-mentioned disadvantages of the prior art.
Another object of the invention is to provide such an apparatus having pairs of electrodes in an array wherein uneven discharge problems at high frequency are eliminated and electromagnetic interference is avoided so that uniform thin films can be formed on all substrates with excellent reproducibility even if substrates having a large area are used.